Scaling in object and edit mode
The object-mode scale is a property of the object; it doesn't actually do anything to the mesh, but rather affects how the mesh is going to fit in the scene.

Scaling in edit mode actually moves the mesh around; it's equivalent to selecting and carefully moving vertices around to make the mesh bigger or smaller.

There are many situation where you don't want to touch the object scale (e.g. physics simulations tend to get mucked up).

If you have changed the object scale, but really meant to scale the mesh, you can "apply the scale" (i.e. apply the object's scale to the mesh, and set the object scale to 1) by ctrla and select "scale" in the popup menu.